Cystic fibrosis (CF) is a lethal disease with a median survival age of 29 years in 1991. Increased survival is associated with a more normal growth curve. However, growth failure continues to be a problem; for example, approximately 40% of the CF individuals in 1990 were below the 5th percentile for normal weight and height. The nutritional deficit in CF is associated with pancreatic insufficiency and with the maldigestion of ingested fat. Pancreatic lipase supplements improve but frequently fail to correct the fat maldigestion. BioMolecular Products has developed a novel, patented lipid-matrix composition (monoglycerides, fatty acids and lysophosphatidylcholine) which represents readily absorbable fat calories. These non-toxic, lipid matrix (named LYM-X- SORB) provided the fat calories for sustaining normal growth of rats from birth until 4 months of age. In this proposed study the improved bioavailability of this lipid matrix in comparison to triglyceride-fat calories will be evaluated in fifteen CF and five normal subjects of approximately 14 years. The bioavailability of these carbon 13-labeled fats will be determined by measuring expired labeled carbon dioxide over 8 hours which requires both absorption and metabolism of the labeled fat. If a marked improvement in the bioavailability of the lipid matrix in comparison to triglycerides were observed in all CF individuals, a lipid-matrix food will be commercially developed for CF.